The antenna used for a radar for detecting the presence of targets at sea and so forth is generally a slot array antenna which sends and receives beams through slots provided on the front panel of a radiator. In this manner, it is possible to improve azimuth resolution by narrowing beam width in the horizontal direction when sending and receiving beams through the slots. Also, 360° degrees of detection is carried out by turning the whole antenna.
In order to detect effectively along a horizontal surface such as the surface of the sea, the beam width must be narrowed down in the vertical direction. However, the radar cannot detect the targets well when the beam width is too narrow, since the beams are jolted out of alignment from targets, for example, when an antenna is fixed on swaying objects such as ships. Accordingly, it is necessary to narrow the beam width in the vertical direction to the extent that the targets can continue to be detected in the presence of some sway.
The whole antenna is enclosed by a housing to minimize external influences such as weather or to provide safety and so forth. However, when beams are radiated outside of the antenna housing and are partly reflected by the antenna housing, interference occurs among these beams, causing side-lobe. Since such side-lobe leads to deterioration of detection accuracy, reduction of the side-lobe is also required.
The antenna 30 is disclosed in Japanese Patent No. 3,634,372 (U.S. Pat. No. 5,757,330) and U.S. Pat. No. 6,819,296. The antenna 30 is described below by referring to FIG. 7 showing the configuration of the antenna 30.
The antenna 30 comprises a radiator 31, slots 32, a transition portion 33, dielectrics 34a-c, antenna housing 35a-b, a support member 36.
The antenna 30 is provided with slots 32 in front of the radiator 31. It is possible to narrow the beam width in the horizontal direction by way of the slots 32. Narrowing the beam width in the horizontal direction can improve the azimuth resolution of the antenna 30. The dielectrics 34a-c are provided through the transition portion 33 ahead of the radiator 31. The height of the transition portion 33 is practically equal to the height of the antenna housing 35a-b made up of the dielectric material, and the transition portion 33 electromagnetically-couples efficiently between the radiator 31 and the dielectrics 34a-c. 
By providing the dielectrics 34a-c ahead of the radiator 31, it is possible to keep the beam from spreading in the vertical direction and to keep the beam narrow in width. That is to say, it is possible to determine desired beam width by changing permittivity and length of the dielectrics 34a-c. 
The dielectrics 34a-c are provided with their longitudinal sides disposed in the direction of the radial axis showing the center of radiation. The dielectrics 34a-c and the antenna housing 35a-b are provided in the form of a layer nearly perpendicular to the radial axis and uniformly spaced in the order of upper surface 35a of the antenna housing, a dielectric 34a, a dielectric 34b, a dielectric 34c, and lower surface 35b of the antenna housing. Furthermore, the dielectrics 34a-c are supported by the support member 36 disposed in the antenna housing 35a-b. In this manner, since the dielectrics 34a-c and the antenna housing 35a-b are provided in the form of a layer, interference occurs (that is, interference among the layers of the dielectrics 34a-c) which is attributable to interference between the upper surface 35a of the antenna housing and the lower surface 35b of the antenna housing, which causes the side-lobe. It is possible to reduce the side-lobe of the beam radiated outside of the antenna housing 35a-b due to interaction of each individual interference.
In addition, although the support member 36 is made of the dielectric material similar to the dielectrics 34a-c, the support member does not contribute greatly to different permittivity, narrowing of the beam width, and reduction of the side-lobe. Therefore, the dielectrics and the support member shall be classified into different factors in view of object and effect.
The antenna 30, however, is heavier due to the presence of dielectrics 34a-c and the support member 36. In general, with regard to the antenna capable of radiating desired beams, its weight is enumerated as one of the important subjects to be improved. The antenna is rotated by a motor for 360° degrees of detection around the antenna. Therefore, as antenna weight increases, the power of the motor must be increased and the strength of a base supporting the antenna must also be increased.
For the antenna 30 described in Japanese Patent No. 3,634,372, the support member 36 is required to dispose and support the dielectrics 34a-c. In addition, it is necessary to form insertion slots for disposing the dielectrics 34a-c in the support member 36, which complicates the manufacturing process of the antenna 30.
The present invention is devised to address the above-mentioned problems. It is an object of the invention to provide an antenna with small side-lobe while narrowing the beam width to a desired width without increasing antenna weight. It is also an object of the invention to reduce the manufacturing requirements and the cost. Furthermore, it is also an object of the invention to improve stability during antenna rotation.